Electrical Checkout Procedures, Test 1 DC Offset

Micro-Tech 1200 Amplifier Service Manual

Electrical Checkout Procedures

GENERAL INFORMATION

The following test procedures are to be used to verify operation of this amplifier. DO NOT connect a load or inject a signal unless directed to do so by the proce- dure. These tests, though meant for verification and alignment of the amplifier, may also be very helpful in troubleshooting. For best results, tests should be performed in order.

All tests assume that AC power is from a regulated 120 VAC source. Test equipment includes an oscilloscope, a DMM, a signal generator, loads, and I.M.D. and T.H.D. noise test equipment.

STANDARD INITIAL CONDITIONS

Level controls fully clockwise. Stereo/Mono switch in Stereo.

Sensitivity switch in 26 dB fixed gain position.

It is assumed, in each step, that conditions of the amplifier are per these initial conditions unless other- wise specified.

TEST 1: DC OFFSET

Spec: 0 VDC, ±10 mV.

Initial Conditions: Controls per standard, inputs shorted.

Procedure: Measure DC voltage at the output connec- tors (rear panel). There is no adjustment for output offset. If spec is not met, there is an electrical malfunc- tion. Slightly out of spec measurement is usually due to U104/U204 out of tolorance.

TEST 2: OUTPUT BIAS ADJUSTMENT

Spec: 300 to 320 mVDC.

Initial Conditions: Controls per standard, heatsink tem- perature less than 40°C.

Procedure: Measure DC voltages on the output module across R02, adjust R26 if necessary. Measure DC voltages on the output module across R21, adjust R23 if necessary. Repeat for second channel.

TEST 3: ODEP VOLTAGE ADJUSTMENT

Spec: Bias Per Chart, ±0.1V DC.

Initial Conditions: Controls per standard, heatsink at

room temperature 20 to 30°C (68 to 86°F). Note: This adjustment should normally be performed within 2 minutes of turn on from ambient (cold) conditions. If possible measure heatsink temperature, if not measure ambient room temperature. Use this information when referencing the following chart. The following is a list of ODEP bias voltages VS. temperature.

°F	°C	V–ODEP	V+ODEP
66	18.9	–10.31	11.41
68	20.0	–10.26	11.36
70	21.1	–10.20	11.30
72	22.2	–10.14	11.24
74	23.3	–10.09	11.19
76	24.4	–10.03	11.13
77	25.0	–10.00	11.10
78	25.6	–9.97	11.07
80	26.7	–9.91	11.01
82	27.8	–9.86	10.96
84	28.9	–9.80	10.90
86	30.0	–9.74	10.84
88	31.1	–9.69	10.79
90	32.2	–9.63	10.73
92	33.3	–9.57	10.67
94	34.4	–9.51	10.61

–ODEP Procedure: Measure pin 6 of U100 and, if neces- sary, adjust R121 to obtain V–ODEPas specified above. Measure pin 6 of U200 and, if necessary, adjust R221

to obtain V–ODEPas specified above.

+ODEP Procedure: Measure pin 6 of U103 and, if neces- sary, adjust R132 to obtain V+ODEP as specified above. Measure pin 6 of U203 and, if necessary, adjust R232

to obtain V+ODEP as specified above.